The present invention relates to a titanium aluminide alloy, particularly to titanium aluminide alloys comprising heavy metals, for example tungsten, or tantalum, and which have a dispersion of borida particles.
Titanium aluminide alloys have potential for use in gas turbine engines, particularly for turbine blades and turbine vanes in the low pressure turbine and compressor blades and vanes in the high pressure compressor. The gamma titanium aluminides provide a weight reduction compared to the alloys currently used for these purposes.
It is known to provide some titanium aluminide alloys with tungsten, such as for example see U.S. Pat. No. 5,296,056, and it is known to provide some titanium aluminide alloys with tantalum, for example see UK patent application GB2,245,593A and UK patent application GB2,250,999A.
It is also known that titanium aluminide alloys may be modified to improve the mechanical properties of the titanium aluminide alloy articles by the addition of boron which forms titanium diboride when the titanium aluminide alloy has solidified. The titanium diboride is an effective grain refiner for the titanium aluminide alloy which improves the castability, mechanical formability and mechanical properties, in particular increased ductility and creep resistance, of the titanium aluminide alloy. See for example U.S. Pat. Nos. 5,284,620, 5,429,796, UK patent application GB2,245,593A and UK patent application GB2,250,999A. In order to provide grain refinement the addition of boron in quantities of about 0.5 to about 2 at % is required
However, it has been found that the addition of boron, or borides, into a tantalum, or tungsten, containing titanium aluminide alloy may result in the formation of precipitate clusters and/or stringers of tantalum boride, or tungsten boride, in the titanium aluminide alloy. This is because the tungsten, or tantalum, in the titanium aluminide alloy reacts with the boron to form the tungsten boride or tantalum boride. The precipitate clusters have a maximum dimension of about 500 xcexcm and are predominantly tungsten boride in tungsten containing titanium aluminides or tantalum boride in tantalum containing titanium aluminides. U.S. Pat. Nos. 5,284,620 and 5,429,796 add the borides into the titanium aluminide alloy in the form of titanium diboride particles and it has been found that the addition of titanium diboride particles to the tungsten, or tantalum, containing titanium aluminide alloys results in the formation of the tungsten boride, or tantalum boride, precipitate clusters.
GB2,245,593A and GB2,250,999A add the boride into the titanium aluminide alloy in the form of elemental boron and it believed that the addition of elemental boron to the tungsten, or tantalum, containing titanium aluminide alloys may result in the formation of the tungsten boride, or tantalum boride, precipitate clusters.
Accordingly the present invention seeks to provide a novel way of adding boron to a heavy metal containing titanium aluminide alloy which at least reduces the above mentioned problems.
Accordingly the present invention provides a method of adding boron to a heavy metal containing titanium aluminide alloy to form a boride dispersion in the heavy metal containing titanium aluminide, comprising:-
(a) forming molten heavy metal containing titanium aluminide alloy,
(b) adding heavy metal boride particles to the molten heavy metal containing titanium aluminide alloy to form a molten mixture, the heavy metal boride particles having the same form as undesirable heavy metal boride precipitate clusters,
(c) cooling and solidifying the molten mixture to form a heavy metal containing titanium aluminide alloy having a uniform dispersion of heavy metal boride particles substantially without the formation of heavy metal boride precipitate clusters.
Preferably step (a) comprises forming molten tungsten containing titanium aluminide alloy,
(b) adding tungsten boride to the molten tungsten containing titanium aluminide alloy to form a molten mixture, the tungsten boride particles having the same form as undesirable tungsten boride precipitate clusters,
(c) cooling and solidifying the molten mixture to form a tungsten containing titanium aluminide alloy having a dispersion or tungsten boride particles substantially without the formation of tungsten boride precipitate clusters.
Alternatively step (a) comprises forming molten tantalum containing titanium aluminide alloy,
(b) adding tantalum boride to the molten tantalum containing titanium aluminide alloy to form a molten mixture, the tantalum boride particles having the same foam as undesirable tantalum boride precipitate clusters,
(c) cooling and solidifying the molten mixture to form a tantalum containing titanium aluminide alloy having a dispersion of tantalum boride particles substantially without the formation or tantalum boride precipitate clusters.
Preferably the titanium aluminide alloy comprises up to 2.0 at % boron, more preferably the titanium aluminide alloy comprises up to 1.0 at % boron and preferably the titanium aluminide alloy comprises more than 0.5 at % boron.
Preferably the heavy metal boride particles added have a size of 1 to 5 xcexcm.
Preferably the density of heavy metal boride precipitate clusters is up to 3 cmxe2x88x922, more preferably the density of heavy metal boride precipitate clusters is less than 2 cmxe2x88x922, more preferably there are substantially no heavy metal boride precipitate clusters.
Preferably the heavy metal boride precipitate clusters have a maximum size of 150 xcexcm, more preferably the heavy metal boride precipitate clusters have a maximum size of 100 xcexcm.
Preferably the titanium aluminide alloy comprises a gamma titanium aluminide.
Preferably the method comprises forming the titanium aluminide alloy into a turbine blade, a turbine vane, a compressor blade, or a compressor vane.
Preferably the titanium aluminide alloy is cast or forged.
The present invention also seeks to provide a heavy metal containing titanium aluminide alloy, the titanium aluminide containing heavy metal boride particles substantially without heavy metal boride precipitate clusters, the heavy metal boride particles having the same form an the undesirable heavy metal boride precipitate clusters, and the titanium aluminide alloy comprises up to 2.0 at % boron.
Preferably the density of the heavy metal boride precipitate clusters is less than 2 cmxe2x88x922.
Preferably the heavy metal boride precipitate clusters have maximum size of 100 xcexcm.
Preferably the heavy metal is tungsten and the heavy metal boride is tungsten boride.
Alternatively the heavy metal is tantalum and the heavy metal boride is tantalum boride.
Preferably the titanium aluminide alloy comprises a gamma titanium aluminide.
Preferably the titanium aluminide alloy is in the shape of a turbine blade, a turbine vane, a compressor blade, or a compressor vane.